Add MjcEqualityAPI as a base class for MuJoCo constraints and MjcEqualityWeldAPI for representing weld constraints.

This change introduces two new USD API schema:
* MjcEqualityAPI - base API for representing MuJoCo equality constraints in USD, this will typically be applied to joints such as UsdPhysicsFixedJoint.
* MjcEqualityWeldAPI - extends MjcEqualityAPI for MuJoCo's weld equality constraints with an attribute for torqueScale.

The usd_decoder is updated to parse UsdPhysicsFixedJoints and convert the new schema attributes into the appropriate mjsEquality data fields.

PiperOrigin-RevId: 856723634
Change-Id: I0982fd3168b643f8e91d87bb5ae5933e526b63a1

include/mujoco/experimental/usd/mjcPhysics/tokens.h

@@ -634,7 +634,7 @@ struct MjcPhysicsTokensType {
   const TfToken mjcSliderSite;
   /// \brief "mjc:solimp"
   ///
-  /// MjcPhysicsCollisionAPI
+  /// MjcPhysicsCollisionAPI, MjcPhysicsEqualityAPI
   const TfToken mjcSolimp;
   /// \brief "mjc:solimpfriction"
   ///
@@ -650,7 +650,7 @@ struct MjcPhysicsTokensType {
   const TfToken mjcSolmix;
   /// \brief "mjc:solref"
   ///
-  /// MjcPhysicsCollisionAPI
+  /// MjcPhysicsCollisionAPI, MjcPhysicsEqualityAPI
   const TfToken mjcSolref;
   /// \brief "mjc:solreffriction"
   ///
@@ -678,8 +678,12 @@ struct MjcPhysicsTokensType {
   const TfToken mjcStiffness;
   /// \brief "mjc:target"
   ///
-  /// MjcPhysicsActuator
+  /// MjcPhysicsActuator, MjcPhysicsEqualityAPI
   const TfToken mjcTarget;
+  /// \brief "mjc:torqueScale"
+  ///
+  /// MjcPhysicsEqualityWeldAPI
+  const TfToken mjcTorqueScale;
   /// \brief "mjc:torsionalfriction"
   ///
   /// MjcPhysicsMaterialAPI
@@ -764,6 +768,14 @@ struct MjcPhysicsTokensType {
   ///
   /// Schema identifier and family for MjcPhysicsCollisionAPI
   const TfToken MjcCollisionAPI;
+  /// \brief "MjcEqualityAPI"
+  ///
+  /// Schema identifier and family for MjcPhysicsEqualityAPI
+  const TfToken MjcEqualityAPI;
+  /// \brief "MjcEqualityWeldAPI"
+  ///
+  /// Schema identifier and family for MjcPhysicsEqualityWeldAPI
+  const TfToken MjcEqualityWeldAPI;
   /// \brief "MjcImageableAPI"
   ///
   /// Schema identifier and family for MjcPhysicsImageableAPI